Tan Delta


Tan Delta Testing Made Easy



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Features and Benefits

Tan delta (Tan δ, TD) testing, also called dissipation factor or loss angle, is used for measuring the degree of deterioration of shielded MV/HV cable insulation. The results reveal how contaminated, damaged, or water tree strewn the insulation has become.

TD measurements are used to satisfy the following objectives:

  • Establish baseline readings on new installations
  • Monitor cable conditions over time
  • Compare the condition of many cables to help prioritize replacement and/or cable injection/rejuvenation programs
  • Determine which additional tests may be warranted

Tan delta testing is performed with the cable off-line. A VLF AC test instrument provides the test voltage to the cable while the TD device records the results. It is a diagnostic test, meaning it is not a VLF AC withstand test where the risk of cable failure is high. With TD testing, cables can be rated as highly, moderately, or slightly degraded. The test voltage is increased slowly in steps while readings are monitored to avoid possible cable failure should the TD numbers indicate severe degradation.

The TD cost is moderate, tests are easy to perform, and the results are easy to interpret.




  • TD-65E 
    0 – 65 kV AC
  • TD-34E – Discontinued